Atlas 5 rocket carrying Air Force spaceplane takes off

Running a day late because of stormy weather, a towering United Launch Alliance Atlas 5 rocket thundered to life and blasted off from the Cape Canaveral Air Force Station Saturday, boosting an unmanned spaceplane toward orbit for a classified military mission.

A United Launch Alliance Atlas 5 rocket boosted an unmanned Air Force spaceplane toward orbit Saturday to kick off a classified military mission. (Photo: United Launch Alliance TV)

Work to fix a leak in a ground helium purge system delayed the launching to the second of two 10-minute window "panes." But the final moments of the extended countdown went smoothly and the rocket vaulted away from Space Launch Complex 41 at 5:46 p.m. EST (GMT-5).

With its Russian-designed RD-180 main engine roaring at full throttle, the Atlas 5 majestically arced away to the East atop a rushing jet of brilliant orange flame, quickly disappearing from view in a clear blue sky.

United Launch Alliance commentary ended 18 minutes after liftoff, after an initial second stage rocket firing. The remainder of the mission is classified and no other details were available.

Perched atop the rocket was the Air Force's second Orbital Test Vehicle, a small 29-foot-long winged orbiter that resembles a smaller version of NASA's manned space shuttle. Also known as the X-37B, the unmanned spaceplane is equipped with its own maneuvering engines, a small payload bay and a solar power boom. Like the manned space shuttle, the OTV uses ceramic tiles to protect the craft from the intense heat of re-entry.

"Launch is a very demanding business and having what appears to be a successful launch is always welcome news," Richard McKinney, deputy under secretary of the Air Force for Space Programs, said in an Air Force post-launch web update. "It is important to remember that this is an experimental vehicle, that this is just the second launch and that we have just started what is a very systematic checkout of the system."

An artist's concept of an X-37 unmanned space plane in orbit. (Photo: Boeing)

NASA came up with the original idea of a small unmanned orbiter, which was built by Boeing Corp.'s Phantom Works division, but fthe program was turned over the Pentagon's Defense Advanced Research Projects Agency, or DARPA, in 2004.

The Air Force took over in 2006. The first X-37B was launched from Cape Canaveral on the program's initial orbital test flight April 22, 2010. The spacecraft spent nearly 225 days in orbit on a blacked-out military mission before gliding to a computer-controlled touchdown at Vandenberg Air Force Base, Calif.

The second X-37B is believed to be carrying a suite of classified experiments. But details about the mission, along with the program's budget, are classified.

"We look forward to testing enhancements to the landing profile," Lt. Col. Troy Giese, X-37B program manager, said in the Air Force web update. "We'll also be looking at the performance of its advanced thermal protection systems and tiles, solar power systems and environmental modeling -- all important system capabilities for a space vehicle that we want to be able to bring back and then re-launch quickly."

He said he expects the second X-37B to remain in orbit about 270 days.

"We may extend the mission to enhance our understanding of the OTV capabilities, especially since the performance data from the first flight suggest that the vehicle could have gone beyond the 270-day requirement," he said.